1. Field of the Invention
The present invention relates to a shift register, and a solid state image sensor using such a shift register and a camera using such a shift register, and more particularly, it relates to a resetting system for a shift register.
2. Related Background Art
FIG. 9 is a circuit diagram showing a conventional XY type solid state image sensor having shift registers and a plurality of photoelectric conversion elements, which was disclosed in Japanese Patent Application Laid-open No. 2002-353430. In FIG. 9, signals read out from a pixel part are successively read and scanned in a vertical shift register 503 and a horizontal shift register 594 and pixel signals are outputted in a time-lapse manner. One pixel is constituted by a photodiode 31, a transfer MOS transistor 32, an amplifying MOS transistor 33, a reset MOS transistor 34 and a selecting MOS transistor 35. A sensor unit is constituted by arranging such pixels in a two-dimensional array.
Further, the vertical shift register 503 performs column scanning of the photodiodes (photoelectric conversion elements) 31 via the election MOS transistors 35 and the horizontal shift resistor 504 performs row scanning of the photodiodes 31 via row selecting MOS transistors 36. Incidentally, since constructions and operations of such vertical shift register 503, horizontal shift register 504 and sensor unit of the solid state image sensor are well-known, detailed explanation thereof will be omitted.
As one of resetting systems for resetting the shift resisters to drive the solid state image sensor, for example, as disclosed in Japanese Patent Application Laid-open No. H06-338198 (1994), a system in which all of stages are reset simultaneously by using an independent reset pulse ΦR is known. An example of an arrangement of such a shift register is shown in FIG. 10.
Incidentally, in this specification, setting to predetermined potential is defined as “reset”, which is used hereinafter throughout the specification and claims.
In FIG. 10, a shift register unit 11 is constituted by first and second clock type inverters 12 and 13 which are connected in series, and a reset MOS transistor 14. The reset MOS transistor 14 comprises a P channel MOS transistor connected between an input node of the second clock type inverter and power potential VDD and is provided in a first stage register. Further, an input signal ΦST is inputted to an input of the first clock type inverter 12 and the reset pulse ΦR is inputted to a gate of the reset MOS transistor 14 of a first stage of the shift resister.
By connecting plural shift register units 11 having the above-mentioned construction as a multi-stage in a longitudinal direction, the shift register is formed. Now, a resetting operation of such a shift register having such an arrangement will be described with reference to a timing chart shown in FIG. 11. Before a high level of the start pulse ΦST for driving the shift register is inputted to the shift register, a low level of an external pulse ΦR for resetting all of stages of the shift register is inputted. The reset MOS transistors 14 for the stage resisters of the shift register are turned ON and the first stage register is reset to the power potential VDD.
However, in order to reset the shift register in the above-mentioned manner, it is required that an additional pulse be given from external, with the result that the number of pads and/or sensor pins will be increased. In order to improve this, it is considered to provide a system of FIG. 12 in which, in the resetting operation of the shift register, without resetting the first stage register, a second stage register and subsequent stage resisters (on and after second stage register) are reset by using the start pulse ΦST for the shift register.
In FIG. 12, a shift register unit 20 of the first stage register is constituted by a first inverter unit 25, a second inverter unit 26 and dummy reset MOS transistor 27. The first inverter unit 25 is constituted by a first switch 21 and an inverter 22 which are connected in series. The second inverter unit 26 is constituted by a second switch 23 and an inverter 24 which are connected in series. The dummy reset MOS transistor 27 comprises an N channel MOS transistor connected between an input node of the first inverter and GND potential and is provided in the first stage register.
A shift register 29 for each of the second stage register and subsequent stage registers is constituted by a first inverter unit 25, a second inverter unit 26 and a reset MOS transistor 28. Each of the reset MOS transistors 28 comprises an N channel MOS transistor connected between an input node of the first inverter and GND potential and is provided in each of the second stage register and subsequent stage registers.
A start pulse ΦST for the shift register is inputted to the inputs of the respective first switches. In order to reset the stage registers of the shift register by the start pulse ΦST for the shift register, the gate of the dummy reset MOS transistor 27 of the first stage register is fixed or held to the GND potential.
The shift register is constructed by connecting the shift register unit 20 of the first stage register having such a construction and the shift register units 29 of the second stage register and the subsequent stage registers as a multi stage in a longitudinal direction. Now, a resetting operation of the shift register having such a construction will be described with reference to a timing chart of FIG. 13.
At the same time when a high level of the start pulse ΦST for driving the shift register is inputted to the shift register, the reset MOS transistors 28 for resetting the various stage registers are turned ON, thereby resetting the various stage registers of the shift register to the GND potential. Further, potentials ΦH1, ΦH2 and ΦH3 of the various stage registers in FIG. 13 correspond to potentials of the first to third stage registers in FIG. 12 and ΦH4 corresponds to the potential of the fourth stage register.
In a case where the shift register is reset in the above-mentioned manner, the resetting of the first stage register of the shift register is not performed. Accordingly, there arose a problem that the first stage register of the shift register becomes unstable to affect the pixel signal.